There's a Mystery Glitch in the Pipe Organ

At Reuter Organ Co. we built real pipe organs. My position was director of production and senior engineer. I had a problem once that I had to solve from 600 miles away. As a musician, I can use my musical skills to analyze various mechanical sounds scientifically. As a licensed amateur radio operator, my electronic skills help to make some problems relatively easy to solve. This particular problem was not so easy.

I received a call from our area representative who told me that the large pipe organ console we had just built and installed was blowing fuses in the three large DC power supplies. The fuses would blow during only about 5 percent of the power-up cycles and all three supplies would blow at the same time.

These supplies delivered power to three independently separate circuits. The representative revealed that it was all or none and whenever it happened, there was a short mild "burp" sound. I asked him to check for any shorts or any other abnormal circuit conditions that would cause over current conditions -- there were none. Everything was normal except for the few times that all three fuses would blow randomly, all at the same time.

Since this console was located almost 600 miles away, I decided to set up the exact circuit conditions in my lab with the one 50 amp and two 35 amp 12V DC supplies delivering power to three similar loads. I connected the supplies to a similar console. I starting latching relays to that in the console that activates the console lights, power supplies, and main air blower controller located in the utility room.

Everything operated correctly with no blown fuses. Since the problem occurred randomly about 5 percent of the time, I continued to cycle the system on and off many times. Finally, I heard a mild burp and all three supplies blew their fuses. I had to repeat this experiment many times to locate the burp sound again. It came from the latching relay.

Now, I could listen for the musical pitch, a low B, from the relay to identify the frequency and duration of the burp, which was 60 Hz for about 1/4 second. But why did it buzz only about 5 percent of the time? Putting a variac on the system, I could replicate the buzz more often with lower voltages. I determined that the spring and armature of the relay mechanically resonated at about 60 Hz and that the inrush current from the largest supply was sufficient to drop the supply voltage just enough to cause the relay armature to drop out for half a cycle.

Once that happened only during the short voltage peak of the AC cycle, the armature would oscillate at 60 Hz, thus operating as a mechanical rectifier causing pulsating 120V DC to continue to the three supplies. While power transformer primaries appear as relatively high impedance devices under little load to AC, they are a virtual dead short at DC. That is why all three supplies always blew fuses at the same time during those 5 percent random times. We fixed the problem by changing out the relay to another brand with an armature/spring resonance at a different frequency than 60 Hz. We also now use switching supplies.

This entry was submitted by Robert J. Vaughan and edited by Rob Spiegel.

Robert J. Vaughan received a BMus in Pipe Organ Performance in 1965 from Bethany College. He built a large three-manual pipe organ just out of college and rebuilt several other pipe organs until he was hired by Reuter Organ Co. in 1969 as a draftsman. Having a keen interest in music, structural engineering, and amateur radio electronics, Robert integrated all three disciplines into his final position with the company as director of production from the late 80s until retirement in 2008. He still does contract design work for the company and likes to restore old wooden tube radios from the 1930s.

Your example of consistancy in components makes so much sense I think it needs to be amplified. Sometimes an equivalent is not equivalent and can lead to faulty assumptions. I think that is also true in terminology. I do not know if that is ever a case in electronics, but in my field different parts of the country call similar things by different names which can lead to confusion when trying to trouble shoot over the phone.

I remember an instance when a customer called me at home about a problem he was having and the conversation quickly turned to jargon and we got the problem solved. When I got off the phone, my wife who had listened to the whole call asked me, "Did he understand what you were saying?" Of course. Why? "Because it did not sound like any English I ever heard before."

Very good diagnostics, and certainly a fault mode that would be quite challenging to predict simply by circuit analysis. So the problem was solved, I hope that there was a design change that came from the dicovery of the problem, and a service note sent to the repair people . That fault mode is not really intuitive. And a quarter of a second is a very short time to hear and evaluate a sound.

So my guess is that there had to be some intuition involved. It is a bit puzzeling about the explanation of how the buzz produced the overload. My guess would have been that it was extending the inrush current time period to where the fuse time delay was exceeded.

Love that example, Nancy. Being able to leverage your professionals skills with your personal passions has to be extremely rewarding and a great way to keep your credentials fresh. Not to mention, the possibilities for another income stream! Enjoy and keep up the great work.

Interesting point, Naperlou. At the beginning, I would imagine it's a pain to have to follow SAP's system requirements. Yet I can see that would give SAP some control over keeping the system working correctly.

Agree, great troubleshooting and teleservicing! This is very similar to another new article, "Super Mistake Caused Super Voltage" It sounds like the lesson for all of us is to really think through relays in power control applications! Don't regard them as a simple on/off contact device.

Many years ago I was engaged to help a client who had a lot of enthusiasm but little experience and had invested a lot of effort in an amazing invention based around a Tandy TRS80 computer. He was attempting to control the intensity of several low-voltage halogen projector bulbs by using simple circuits originally intended to vary the speed of a mains-voltage electric drill. This too was very temperamental and would sometimes work after a fashion but would eventually blow the fuses very spectacularly, accompanied by loud grunts from the transformers that fed the bulbs. I pointed out that such simple dimmer circuits had no protection against "half-cycling" wherein the AC delivered to the transformer acquired a significant DC component, with obvious results. The "universal motors" in old electric drills don't mind the ragged waveform, nor would a mains-voltage filament lamp. I completely redesigned the circuit to use low-voltage DC with PWM control of the brightness and the problem was solved.

Agree with naperlou. Work with the equipment and understand it. And everything is significant; the 'burp' being a key clue in this story. This reinforces one key troubleshooting theorem I applied first as a technician and then after I got my EE degree: the problem that kicks your butt the hardest usually has the simplest solution, in this case replace the relay.

I liked the fact the entire system was rebuilt and the fact that it wasnt a repeating failure. It was a try and try again to find the failure. Sometimes the textbooks make it sound so simple with the massless ropes and frictionless surfaces. Often it takes a lot of hands on time in the lab to solve a problem.

Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.